1. Field of the Invention
The present invention relates generally to implantable blood access devices of the type used to perform hemodialysis, and more specifically, to an implantable vascular access fistula providing both acute external access, as well as chronic access via percutaneous cannulation.
2. Description of the Prior Art
Hemodialysis is now a commonly practiced method of treating patients suffering from renal failure. Hemodialysis machines serve to remove life-threatening chemicals from the blood stream, when the kidneys themselves can no longer effectively remove such chemicals. In order to perform hemodialysis, access must first be obtained to the blood flow system.
Various blood access devices have been developed to facilitate direct access to the vascular system. The Scribner arteriovenous shunt described by Quinton et al., "Cannulation of Blood Vessels For Prolonged Hemodialysis", Trans Am Soc Artif Organs 6: 104-113, 1960, includes a length of Silastic tubing having catheter tips at each end. The catheter tips are inserted into an artery and a vein below the skin, while the Silastic tubing extends primarily external to the skin, except at the ends thereof where it passes through the skin. Such external shunts are plagued by infection at the entrance sites where the Silastic tubing passes through the skin, as well as by clotting problems. External shunts further pose an inconvenience to the patient because of the extra care which needs to be taken to avoid injury or dislodgment of the Silastic tubing.
Many of the problems arising from the external Scribner shunt are avoided by the formation of a subcutaneous autogenous arteriovenous fistula, such as the type known as the Brescia-Cimino fistula, which is formed using the cephalic vein and the radial artery in the forearm. Arterial pressure causes contiguous veins to dilate and provide sites for venipuncture. However, such a fistula may not be punctured for several weeks because of the time required for the walls of the dilated vein to thicken.
Several atraumatic vascular access devices are known wherein repeated percutaneous needle punctures may be avoided. One such device is known as a Buselmeier shunt and is described in U.S. Pat. No. 3,826,257 issued to Buselmeier. The Buselmeier shunt, like the Scribner shunt, provides a length of Silastic tubing having vessel tips at each end to be cannulated into an artery and an adjacent vein. In the Buselmeier shunt device, the shunt tube is U-shaped and is intended to be implanted below the skin. Extending from the Silastic shunt tube are one or two access tubes which project outwardly through the skin where a closure plug is installed to seal off the access tubes. The closure plugs are removed when dialysis is to be perfomed.
Button shunt devices are also known wherein a skin level port is provided to avoid the need for percutaneous needle puncture One such button shunt device is commercially available under the registered trademark "HEMASITE" from Renal Systems of Minneapolis, Minn. This device is described in U.S. Pat. No. 4,496,350 issued to Cosentino and assigned to Renal Systems. The "HEMASITE" device includes a T-shaped body having lengths of expanded polytetrafluoroethylene (PTFE) tubing secured to opposing ends thereof for the purpose of interconnecting the device between an artery and a vein. Extending upwardly from the T-shaped body is a stem which protrudes permanently through the skin. The stem includes a septum which normally precludes blood flow outwardly therethrough. The external end of the stem is normally closed by a removable cap. When access is desired, the cap is removed and a dual-needle assembly that mates with the stem is inserted therein to pass through the septum for communicating with the bloodstream.
Another button-type shunt is commercially available under the trademark "DiaTAP" from American Bentley, a subsidiary of American Hospital Supply Corporation of Irvine, Calif. This device is described in U.S. Pat. No. 4,164,221 issued to Bentley et al. and assigned to Bentley Laboratories. The device includes a stem which extends upwardly from a length of expanded PTFE tubing, passing just above the skin at its upper end to provide a permanent skin level port. A plug normally fits within the stem to prevent blood from flowing outwardly therethrough. A removable cap retains the plug within the stem until access is desired. A special valved connector is coupled to the external end of the stem for controlling blood flow therethrough when access is desired.
While the aforementioned atraumatic vascular access devices avoid percutaneous needle puncture, such devices still present the risk of infection inherent in any device which permanently passes through the skin. Moreover, discontinuities within the blood flow path provided by such prosthetic devices often cause a buildup of clotted blood which may restrict or ultimately seal off the flow of blood therethrough, resulting in thrombosis. Such discontinuities arise, for example, in the "HEMASITE" device wherein the T-shaped body is joined with the surrounding PTFE tubing, and in the "DiaTAP" device at the point where the stem is secured to the expanded PTFE tubing.
Apart from the atraumatic vascular access devices discussed above, prosthetic vascular grafts, or bridge fistulas, are also known wherein the graft is implanted entirely below the skin to provide a readily palpable conduit that can easily be cannulated by a needle to obtain access for hemodialysis. Such prosthetic vascular grafts are often connected within the vascular system between the distal radial artery and the cephalic or basilic vein. Each end of the fistula is anastomosed in an end-to-side fashion to the respective artery or vein. The most commonly used material to form prosthetic vascular grafts is expanded polytetrafluoroethylene (PTFE) U.S. Pat. No. 4,187,390 issued to Gore discloses a process which may be used to produce highly porous, expanded PTFE structures. PTFE vascular grafts are commercially available under the registered trademark "IMPRA" from IMPRA, Inc. of Tempe, Ariz. Methods of implanting such prosthetic vascular grafts are described in Bennion et al., "Hemodialysis and Vascular Access", Vascular Surgery, pp. 625-662, 1983.
When a physician decides to implant a PTFE vascular graft to provide vascular access following renal failure, the physician must also decide whether or not to simultaneously place a subclavian or jugular vein catheter within the patient to provide temporary acute access to the blood system until the PTFE vascular graft is well-healed. PTFE grafts are highly porous, and surrounding body tissues ultimately grow into the walls of the PTFE vascular graft, resulting in a more natural blood vessel. However, the healing process, during which tissue ingrowth occurs, takes approximately two weeks in most patients. Cannulation of the PTFE vascular graft prior to healing often results in hematoma formation, false aneurysm, leaking of blood from the puncture site, and early fistula failure.
The risks of premature cannulation of the graft can be avoided by short-term placement of a catheter to gain temporary vascular access while the graft heals. However, placement of a subclavian or jugular vein catheter within the patient also presents certain risks. Obviously, the need to perform two surgical operations simultaneously instead of only one complicates surgery. Moreover, placement of a subclavian or jugular vein catheter may result in hemothorax, pneumothorax and other complications. Recently, subclavian vein thrombosis has become a major concern because its presence limits the number of potential fistula sites.
It is an object of the present invention to provide an implantable blood access device for use in performing hemodialysis which avoids the use of tubes or other skin level ports that permanently pass through the skin, in order to minimize long-term infection risks.
It is another object of the present invention to provide an implantable access fistula that permits acute external access to the vascular system for permitting the fistula to become well-healed, while further permitting chronic access to the vascular system by percutaneous cannulation of the fistula.
It is yet another object of the present invention to provide such an implantable fistula which includes a smooth, continuous flow surface to minimize the risk of thrombosis.
Still another object of the present invention is to provide a PTFE vascular graft wherein acute access may be obtained without prematurely puncturing the walls of the graft, and without the need to place a subclavian or jugular catheter within the patient.
A further object of the present invention is to provide a PTFE tube structure which is adapted to be cannulated subcutaneously in the manner of conventional PTFE vascular grafts, while simultaneously including a hardened portion through which acute access may be obtained without disturbing the healing of the PTFE tube and without creating any discontinuities therein.
These and other objects of the present invention will become more apparent to those skilled in the art as the description thereof proceeds.